Bidirectional selection affecting life span will be applied to a hybrid population of D. melanogaster. Direct selection for increased or decreased life span will be applied to one population, while selection for reproduction early or late in life will be applied to a replicate population. A variety of standard laboratory or wild caught strains may be used for the hybrid population. The heritability and variation in life span of the strains selected for use will be measured and progeny tested for evidence of hybrid dysgenic effects before strains are hybridized.